1. Field of the Invention
The present invention relates to a disk brake device for a motorcycle. Furthermore, the present invention relates to a caliper structure for a front wheel brake, which is suitable for a motorcycle including a link-type front suspension.
2. Description of Related Art
A general disk brake device for a motorcycle includes a disk brake mounted on the hub of a wheel supported on the vehicular body side through an axle; a brake caliper for braking the disk brake; and a caliper bracket, turnably supported on the axle through a bearing, for supporting the brake caliper. In such a disk brake device, the mounting position of the disk brake is often offset from the bearing position of the caliper bracket. In some cases, however, it is desired to adopt a non-offset arrangement type for the disk brake device.
Japanese Utility Model Publication No. Hei 2-20157 discloses a disk brake device for the rear wheel of a motorcycle, which adopts such a non-offset arrangement type. In this device, a hub end plate has a connecting projection at a central portion formed into an approximately rectangular shape and mounted on a side surface of a hub of the rear wheel; a thick boss having a connecting recess to be fitted with the above connecting projection is mounted to an inner peripheral portion of the disk brake; and the hub end plate is integrally rotatably connected to the disk brake by fitting the connecting projection in the connecting recess.
A bracket boss of a caliper bracket is supported on an axle through a needle bearing, and is relatively rotatably fitted inside the above boss of the disk brake through a ball bearing provided on the outer peripheral portion of the bracket boss. The needle bearing and the ball bearing are coaxial with each other, and are overlapped with each other at a position through which the rotational plane of the disk brake passes.
With this configuration, since the outer periphery of the bracket boss of the caliper bracket is fitted inside the boss of the disk brake and the boss of the disk brake is supported by the hub, it is possible to enhance the supporting rigidity of the boss of the caliper bracket, and since the disk brake is disposed in such a manner that the rotational plane thereof passes through the bearing, it is possible to obtain the non-offset arrangement type structure.
While it is effective to extend the span of a bearing portion supported on the axle for enhancing the supporting rigidity of the bracket boss, a needle bearing has a limitation in sufficiently extending the span thereof. In the above example using the needle bearing, to enhance the supporting rigidity of the bracket boss having a relatively short span, it is required to provide the hub end plate and the boss of the disk brake, each of which is very heavy. This increases the weight of the structure and the number of parts, and complicates the structure. Also, the outer periphery of the bracket boss, which must be rotatably supported by the boss of the disk brake, is required not to be loosely fitted but to be accurately fitted in the boss of the disk brake. As a result, the bracket boss must be finely machined and finely assembled in the disk brake.
With regard to the caliper structure in the related art, U.S. Pat. No. 4,775,163 discloses a link-type front suspension for a motorcycle. The link-type front suspension includes a pair of right and left front forked portions which extend in the vertical direction with a front wheel located therebetween. Upper ends of the front forked portions are steerably supported by a body frame. In addition, a pair of right and left push rods are disposed substantially in parallel to the front forked portions; pivot links are rotatably mounted on lower ends of the front forked portions; lower ends of the push rods support an axle of the front wheel; and damping devices connect upper ends of the push rods to the body frame.
In the above related art front suspension, one end of a caliper bracket is turnably supported around the axle of the front wheel; a caliper for a front wheel brake is mounted on the other end of the caliper bracket; and the caliper bracket is connected to the front forked portion with a torque link.
Incidentally, such a caliper is generally of a pin-slide type in which a braking force is transmitted from the caliper to the caliper bracket and is further transmitted to the front forked portion through the torque link. As a result, the caliper bracket functioning as a braking force receiving member is required to have a high rigidity, and thereby it is increased in weight and it is required to be formed from an expensive high grade material. Furthermore, the caliper bracket is generally offset in the lateral direction from the center of the vehicular body, and accordingly, if the caliper bracket is increased in weight, it is difficult to take a balance of the caliper structure on the right and left sides and to lower the unsprung weight.